1. Field of the Invention
The present invention relates to an optical demultiplexer primarily for use in wavelength division multiplexing optical communications, and a method of assembling an optical demultiplexer in optical axis alignment.
2. Description of the Related Art
Heretofore, it has been customary to assemble an optical demultiplexer by securing its various optical components to a jig by adhesive bonding for optical axis alignment, centering and fixing the optical components secured to the jig on a planar board by way of active alignment, and finally placing the planar board, to which the jig holding the optical components is fixed, in an opaque case of stainless steel or aluminum.
According to another conventional practice, various optical components are inserted into a single metal tube, and centered and fixed in position within the metal tube by way of active alignment while their positions are being observed with either an observing tool inserted into the metal tube through an open end thereof or a naked eye through an open end of the metal tube. The term xe2x80x9cactive alignmentxe2x80x9d refers to a process of centering the optical components by entering light via an optical fiber and moving the optical components slightly for an optimum position (for a maximum optical output power) while monitoring an optically coupled state of the optical components.
With the former assembling technique, it is difficult to align the optical axes of the optical components with each other because of variations of the outside diameters of the optical components and variations of the machining accuracy of the jig which is used to achieve optical axis alignment, and it is also difficult to set surfaces of the optical components or various optical elements to desired angles with respect to the optical axis of the optical demultiplexer or with respect to each other. Even after the optical components have been bonded to the planar board, since the setup of the optical components is not held in axial symmetry with respect to the optical axis of the optical demultiplexer, the optical performance of the optical demultiplexer tends to be unstable when subjected to temperature changes and vibrations.
The latter assembling practice has been disadvantageous in that when the optical components are aligned in the metal tube, they are liable to suffer angular deviations from the optical axis of the optical demultiplexer, resulting in a failure to achieve designed optical performance after the optical components are bonded in position. Furthermore, inasmuch as the optical components in the metal tube cannot directly be observed from outside of the metal tube, it is difficult to make accurate adjustments for centering the optical components for alignment and mechanically positioning the optical components in desired positions.
It is therefore an object of the present invention to inexpensively provide an optical demultiplexer whose optical components are aligned easily and highly accurately with each other without active alignment that has heretofore been required for highly accurate centering or adjustments and hence within a reduced period of time in the absence of active alignment.
According to the present invention, there is provided an optical demultiplexer comprising a plurality of tubes combined in axially slidably interfitting relationship to each other, the tubes being permeable to light, an input optical fiber, a collimator lens, and a diffraction grating, the input optical fiber, the collimator lens, and the diffraction grating being mounted on the tubes. Each of the tubes preferably comprises a hollow cylindrical tube.
The optical fiber, the collimator lens, and the diffraction grating are fixed to end faces of the tubes, preferably by adhesive bonding.
Each of the tubes is made of a material selected from the group consisting of a transparent material, a translucent material, and a colored material.
Each of the tubes may have a ground outer surface if they could be axially slidably interfitted to each other.
Preferably, each of the tubes has a coefficient of linear expansion which is at most 50xc3x9710xe2x88x927/xc2x0 C.
The diffraction grating preferably comprises a reflective diffraction grating and is made of a material which is the same as each of the tubes.
The optical demultiplexer further comprises a detector for detecting light introduced from the input optical fiber, applied through the collimator lens to the diffraction grating, demultiplexed by the diffraction grating, and converged by the collimator lens. Preferably, the detector is positioned in conjugate relationship to an end of the input optical fiber fixed to the end face of one of the tubes. The detector comprises a photodetector array for detecting focused spots of the light demultiplexed by the diffraction grating and converged by the collimator lens.
The tubes may include three tubes, the input optical fiber, the collimator lens, and the diffraction grating being mounted respectively on the three tubes. The tube on which the input optical fiber is mounted and the tube on which the diffraction grating is mounted are slidably fitted over respective opposite ends of the tube on which the collimator lens is mounted.
Alternatively, the tubes may include two tubes, the input optical fiber being mounted on an end face of one of the two tubes, the collimator lens and the diffraction grating being mounted on respective opposite end faces of the other of the two tubes. The tube on which the input optical fiber is mounted is slidably fitted over the tube on which the collimator lens and the diffraction grating are mounted.
According to the present invention, there is also provided a method of assembling an optical demultiplexer having an input optical fiber, a collimator lens, and a diffraction grating, comprising the steps of preparing a plurality of tubes dimensioned to be combined in axially slidably interfitting relationship to each other, the tubes being permeable to light, installing the input optical fiber, the collimator lens, and the diffraction grating on the tubes, bringing the tubes into axially slidably interfitting relationship to each other, and sliding the tubes relatively to each other to set the distance between reference points related to the input optical fiber, the collimator lens, and the diffraction grating to a predetermined value for thereby achieving centered alignment.
If the tubes include three tubes, then the optical demultiplexer is assembled for centered alignment by installing the input optical fiber, the collimator lens, and the diffraction grating on the three tubes, bringing the tubes into axially slidably interfitting relationship to each other, and sliding the tubes relatively to each other to set the two distances between reference points related to the input optical fiber, the collimator lens, and the diffraction grating to respective predetermined values for thereby achieving centered alignment.
If the tubes include two tubes, then the optical demultiplexer is assembled for centered alignment by installing the input optical fiber, the collimator lens, and the diffraction grating on the two tubes, bringing the tubes into axially slidably interfitting relationship to each other, and sliding the tubes relatively to each other to set the distance between reference points related to the input optical fiber, the collimator lens, and the diffraction grating to a predetermined value for thereby achieving centered alignment.
Specifically, after the optical components, i.e., the input optical fiber, the collimator lens, and the diffraction grating have been installed on the tubes, the tubes are brought into slidably interfitting relationship to each other. Then, the tubes are axially slid relatively to each other to adjust the distances between the input optical fiber, the collimator lens, and the diffraction grating, and angularly slid relatively to each other to adjust the relative angles between the input optical fiber, the collimator lens, and the diffraction grating. Such distance and angular adjustments can be made by passive alignment which, rather than applying a light beam to the optical demultiplexer, achieves optimally aligned positions by moving the tubes into alignment with alignment markers or reference planes as observed from outside of the tubes. The time needed to perform the passive alignment is shorter than the time needed to perform active alignment, and s the optical demultiplexer may be manufactured and assembled inexpensively.
If the tubes are axially symmetrical with respect to the optical axis, e.g., comprise hollow cylindrical tubes, then the optical performance of the optical demultiplexer is stable when subjected to temperature changes and vibrations after the optical components are fixed to the tubes. If the diffraction grating is made of the same material as the tubes, then the diffraction grating has the same coefficient of linear expansion as the tubes, and hence is thermally stable.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.